1. Field of Invention
This invention relates to potentiometer detection and measurements, and more particularly to the detection and measurement of halide ions, including chloride ion, Cl.sup.-, bromide ion, Br.sup.-, and iodide ion, I.sup.-.
2. Prior Art Statement:
The standard mercury pool calomel electrode has been known since 1890, when it was introduced by Ostewald. The history, theory, and operation of this electrode are amply set forth in Reference Electrodes, Theory and Practice, edited by Ives and Janz, 1961, Chapter 3. The construction of this electrode consists in part of placing calomel (mercurous chloride, Hg.sub.2 Cl.sub.2) on top of a mercury pool. By placing an internal electrolyte, such as concentrated KCl or the like in contact with the pool and the calomel, a standard calomel reference electrode is created. By using the solution of interest as the internal electrolyte, a variable potential electrode is set up, but the use of such electrodes has not been widespread. (See, Reference Electrodes, Ch. 3, supra). The central and pervasive limitation of this electrode, together with its bromide ion and iodide ion counterparts, is structural: the liquid mercury pool makes ion-selective use of this electrode extremely difficult.
A substantially different approach to measurement of chloride ion, bromide ion, and iodide ion has been disclosed by Ross, et al in U.S. Pat. No. 3,563,874, issued Feb. 16, 1971. In this patent, a substantially imporous membrane of a mixture of silver sulfide and a silver halide selected from the group consisting of silver iodide, silver bromide and silver chloride was disclosed to be selectively sensitive to the respective halide ions.
J. Sekerka and J. F. Lechner in J. Electroanal. Chem., 57, 317 (1974) describe a chloride ion-selective electrode having an active membrane composed of a mixture of mercuric sulfide, HgS, and mercurous chloride, Hg.sub.2 Cl.sub.2 (calomel). A near parallel effort by Paul K. C. Tseng and W. F. Gutnecht as described in Analytical Letters, 9 (9) 795-805 (1976) has resulted in the production of a bromide ion-selective electrode having an active membrane composed of HgS and Hg.sub.2 Br.sub.2. See also, J. Sekerka and J. F. Lechner, J. Electroanal. Chem., 69, 339 (1976). More recently, G. B. Marshall and D. Midgley have described a solid state chloride ion selective electrode with a membrane composed of a compressed mixture of mercury (II) sulphide and mercury (I) chloride, Analyst, May, 1978, Vol. 103 p. 438. This electrode has been found suitable for determining chloride at concentrations as low as 10.sup.-7 mol/1.
V. Hannema, G. J. van Rossum, and G. den Boef published a paper in November, 1970 On the Use of the Mercury Electrode in Chelatometric Potentiometers Titrations, Z. Anal. Chem., 250, 302-306 (1970). A mercury electrode is described in the form of a solid gold rod. The rod is covered with plastic tubing except at the ends. The lower end is covered with mercury and the electric connection is made at the other end. Before every titration the mercury electrode is superficially cleaned with 14 M HNO.sub.3, rinsed with water, re-coated with mercury by dipping it into metallic mercury, rinsed with water, dipped into 0.1 M EDTA and again rinsed with water. The Hannema electrode is sensitive to mercury ion concentrations, not chloride ions. The Hannema electrode, in sharp contrast to the present invention, is not formed of any intimate mixture of gold, mercury, and mercurous halide salt. As a result, perennial cleaning and re-coating with mercury prior to each determination of mercury metal ions is required.